Sintered high energy permanent magnets



NOW 1952 R. J. STUDDERS ETAL. 2,617,723

SINTERED HIGH ENERGY PERMANENT MAGNETS Filed May 4, 1949 DEMAGNET/ZINGFORCE-H'OERSTEDS Inventors: Robert J. Stuclclers, Doiph 6 Ebehng, byTheir Attown ey.

Patented Nov. 11, 1952 UN'IE ST SINTE'RED HIGH ENERGY PERMANENT MAGNETSApplication my 4, 1949, Serial No. 91,324

7 Claims.

The present invention relates to sintered high energy Alnico magnetsprepared by powder metallurgy methods. It is more particularly concernedwith sintered high energy permanent 'magnets characterized by a massivecrystalline structure and a BH max. value equal to or higher than the BHmax. values of the best cast anisotropic Alnico magnets.

Both cast and sintered Alnico magnets of various compositions butessentially containing alu- .minum, nickel, cobalt, and iron, have beenon the market for-same years.

As compared with the cast products, the sintered magnets, prepared, Iforexample, in accordance with the teachingsof Howe Patents 2,192,743 and2,192,744

have been characterized by the finegrain structure normally ioundinsintered products made .by the :usual powder metallurgy techniques.

The :sintered products have also been characterized "by somewhat lowermagnetic properties than the'correspon'ding cast alloys due to theimpracticability of obtaining maximum physical density inthe sinteredalloys.

- -While'mostof the Alnico magnets have been :magneticallyisotropi anoutstanding exception arethe castanisotropic magnets preparedinaccordance with the teachings of the JonasPatent 2,295,082. The castmagnet alloys-which are described in the Jonas patent as iron basealloys having a cobalt content of 16 to 30 percent, a

nickel content of 12m 20 per cent -and'an aluminum content-016 to 11percent with or without other additions such as up to 7 copper, arerendered anisotropic byheat treatment (cooling) in a magnetic field. Theheat treated products exhibit BH maxvalues-in one direction at least 50per cent, and generally over 100 percent, higher'than'those of thesame'alloy-Which has not been -so heat treated. Representative of thesecast anisotropic magnets is the commercially available magnet known asAlnico5. This magnet alloy consistsof about *8 per cent -aluminum, 14percentnickel, 24 per cent cobalt, '3 per cent copper, balancesubstantially iron,

Heretofore, it has not been found possible'to prepared rsin'teredanisotropic -Alnico magnets comparable with the cast anisotropic magnetsby the simple duplication -in-the sintered product of the chemicalcomposition of the cast magnet. This indicates that factors other thanchemical composition influence the efiectiveness of the magnetic fieldtreatment in imparting superior available energy products to the alloy.

The present invention has as a primary object the preparation ofsintered anisotropic Alnico magnets having BI-I max. values as highor-higher than those'of a cast magnet of the same composition. I

Another object of the invention is to provide a high energy sinteredAlnico magnet characterized by a massive crystalline structure.

These, and other objects which willbecome apparent from the followingdescription taken in connection with the accompanying drawing in whichthe single figure includes demagnetization curves of magnets prepared inaccordance with thepresent invention.

The invention will best be understood after first considering thepractices heretofore vemployed in the manufacture of sintered magnets,which practices are more-fully described in the above-mentioned Howepatents.

In accordance with those practices, whereby dense, fine-grained alloysof superior strength could be produced to closetolerances, oxidation ofthe aluminum is prevented by adding this element as a pre-alloy ofaluminum andiron, cobalt or nickel. A pre-alloy of 50 per cent aluminumand 50 per cent iron is generally-used because it is brittle and easilydisintegrated by .meltin point ofthe aluminumalloy v(aluminumiron')component.

The advisability of using aluminum-mickel alloys or aluminum-iron alloysother than the 50:50 alloy has also been investigated,the-conclusionbeing thatjfor acceptable results the prealloy should havea melting point 'belowlbnt close-tothat ofthe ,finalalloy. For example,.G. RitzanfiWissfiVerofi aus den Siemens-Werken, Werkstoff ,SOndBrheft.37 (1,940), concludes that suchother .pre-alloys, as may be .used,should-be t os which ive a -1 qu d. phas -a hesinte ng temperature andwhich are not characterized by a maximum melting point. The sameconmetals.

clusions are presented by S. J. Garvin in his article entitled SinteredPermanent Magnets, in Engineering June 6, 1947.

The commercial sintered Alnico products and those obtained by followingthe teachings of these investigators have been characterized by afinegrained structure with a grain size the order of the size of themetal particles employed in making the products.

In accordance with the present invention, there has been providedsintered Alnico magnets differing from the prior art products in havinga massive, and in some cases, a single crystal structure. In addition,the anisotropic magnets of the present invention are characterized byenergy values in a principal direction substantially higher than thoseof cast anisotropic magnets of the same composition.

These results are obtained by a sintering process which primarilydiffers from the prior art processes in that it includes theintroduction of the low melting point constituents, particularly theoxidizable low melting constituent, i. e., the aluminum, in the form ofa master alloy or prealloy having a melting point at or above, ratherthan below, the sintering temperatures.

While the invention is not limited thereto, it will be particularlydescribed with reference to the preparation of sintered anisotropicmagnets of the composition given in the above-mentioned Jonas patent,specifically a sintered anisotropic magnet of the same chemicalcomposition as Alnico 5.

In preparing the sintered products of the present invention, thepowdered metal constituents are mixed in the required proportions withthe aluminum constituent being added in the form of a high melting pointalloy of cobalt, nickel or iron. If desired, the copper which also meltsbelow the sintering temperature (1200 to 1400" C.) may also be added inthe form of a high melting point alloy with one of the remainingHowever, when the total quantity of copper is low, 6. g., about 35%,this procedure is not necessary, probably due to the fact that duringsintering the unalloyed copper may be continuously consumed in theformation of a high melting point alloy so that the net result is in'fact the formation of a copper alloy having the melting pointrequirements specified hereinbefore. v Except for the introduction ofthe low melting point metals, such as, the aluminum in the form of analloy having no molten phase below the sintering temperature, theproducts of the present invention are prepared in substantially the samemanner as that usually employed in making sintered Alnico magnets inaccordance with the above-mentioned Howe patents. The finely dividedaluminum alloy is mixed with the other finely divided metal ingredientsto provide a mixture of the intended composition and the mixture pressedinto the desired shape.

The pressed products are sintered in a hydrogen atmosphere attemperatures of from about 1200 to 1400 0., preferably below but closeto the melting point of the alloy. The time required for the sinteringaction will, of course, depend upon the size of the furnace load andsize of the pressed pieces. Ordinarily, the sintering time will be fromabout two to five hours. The sintered material, which will now becharacterized by a crystal structure which is a ten to twenty thousandfold increase in the crystal size over the starting material, i. e., themetal powder, can then be subjected to the magnetic field treatmentdescribed in the Jonas patent to obtain an anisotropic magnetic product.The heat treatment in the magnetic field is preferably carried out bywithdrawin the material from the sintering zone of the furnace at atemperature of about 1250 C. and controlling its cooling cycle in amagnetic field of proper field strength. Further low temperaturetreatments may be applied as described by Jonas.

This process, insofar as the production of a massive crystal structureis concerned, is better than any known process for the commercialproduction of large crystals by casting of the molten alloy. Thebenefits of the massive etype crystal structure, which often is a singlecrystal, are readily apparent from the test results set forth in thefollowing table.

MASSIVE CRYSTAL SPECIMENS No. B, H BH max. At B 12, 500 655 5. 64x10 10,400 12, 550 670 5. 96 (10' 10,450 12,050 635 5 l8 l0 9,600 12, 700 69010, 200

MATERIAL MADlgiaVlTl-l LOW FINE GRAINED MELTING MASTER ALL In the abovetable, the specimens 1-4 inclusive had the same composition-as Alni-co 5and were prepared in accordance with the present invention using apro-alloy of 25 per cent aluminum, balance cobalt. Specimen 5 was afine-grained sintered product of the same composition prepared by use ofthe low melting 50:50 Fe-Al prealloy. The demagnetization curves ofspecimens 3, 4 and 5 are shown in the accompanying drawing, from whichit will be seen that there is a tremendous increase in BH max. in thetransition from the fine-grained structure of specimen 5 to a massivecrystal structure of specimens 3 and 4. It is to be noted also that theBH max. values for specimens 1-4 inclusive are all higher than the BHmax. value of cast Alnico (Alnico 5) of-the same composition. j,

The aluminum pre-alloys employed in' the practice of this inventioninclude those which can be selected, for example, from the phase.

diagrams of the binary Fe-Al, Co-Al, and Ni-Al alloys as having nomolten phase below the sintering temperatures. The useful alloys includethe aluminum-cobalt alloys containing from about 67 to per cent byweight of cobalt, the aluminum-nickel alloys containing about 64 to 80per cent by weight nickel and the aluminumiron alloys containing up to18 per cent aluminum. All of these designated alloys are characterizedby freedom from a molten phase below 1400 C. For practical reasons, thealuminumcobalt alloys are preferred.

The advantages of the present method in which the aluminum is added tothepowdered mixture in the form of a high melting point alloy are verypronounced. In addition to the higher BH max. values of the anisotropicproducts along the crystallographic face, all of the productshave asurface which is comparable with the best surface obtainable byprecision casting methods. In addition, the products have a physicaldensity which is higher than that of the sintered maspecific gravityisabout 5-6% lower than that of ;a cast magnet .of the same composition,the magnetic properties are comparable with the best cast magnet and aremuch superior to the magnetic properties of the usual commercial cast=Alm'co magnets.

, It has also been noted that there is an improvement in the physicalstrength of the sintered materials prepared as described herein,.possibly because, in the absence of a molten phase, there is lesstendency for the aluminum -to oxidize so that the impurities are morerestrained in their movement and are not expelled -:t o the grainboundaries. This result is to be compared with the usual cast orsintered Alnicos in which the molten phases allow the impurities .tocongregate and form grained boundaries which can act as restrictionsagainst further growth of the crystals.

It is believed that at'least three things contribute to the formation ofthe massive crystal structure in the present product. First, due to thecondition of the aluminum master alloy surface, oxidation of the powderis at a minimum. Secondly, a somewhat longer sintering time of at leasttwo hours and the use of a sintering temperature which approaches withinl5-50 of the melting point of the entire composition appears to effectthe growthof the massive crystal structure under conditions which do notfavor the congregation of impurities. Thirdly, the absence of any moltenaluminum-bearing phase greatly reduces any tendency toward oxidation ofthe aluminum during sintering.

The following experiment illustrates the difference in the resultsobtained by employing a high melting point aluminum-cobalt alloy ascompared with a lower melting point aluminumcobalt alloy. In thisexperiment, two mixes were made up of identical compositions, oneemploying a 50:50 aluminum-cobalt master alloy and the second a 25:75aluminum-cobalt master alloy. Both alloys were reduced to a minus 200mesh powder under conditions which were designed to prevent anysignificant oxidation of aluminum components thereof.

These binary alloys were then made into powder mixes of the Alnico 5composition. Test bars were pressed from the mixes. One bar from eachmix was placed in a sintering boat with the bars placed side by side.These boats were then passed through a sintering furnace at a rate ofinches per 2.25 hours at a temperature of 1380-1385 0.

Upon examination of the sintered products, it was found that the barsfrom the mix containing lower melting 50:50 aluminum-cobalt alloy wereseverely distorted, melted, and were full of holes. The bars from the:75 aluminumcobalt mix had a very smooth surface with practically nodistortion. Upon fracturing the bars, it was found that each of the barsfrom the 25:75 aluminum-cobalt mix was characterized by a massivecrystal structure, while every bar from the 50:50 aluminum-cobalt mixwas essentially fine-grained and comparable in this respect to thepreviously known sintered Alnico magnet alloys. The massive crystalstructure products possessed BH max. values of the order of specimens1-4, as set forth in the foregoing table, whereas the 50:50aluminum-cobalt products possessed BH max. values comparable with the BHmax. value of specimen 5.

A further aspect of the present invention comprises a modification ofthe process whereby the massive crystalline structure can be obtainedwith an orientation of the crystal in the direction most favorable tothe influence of the directionalizing magnetic field applied during theheat treatment in the preparation of anisotropic products.

Alnico compositions of the type with which the present invention isconcerned have been found to possess a body centered cubic crystallattice and the most desirable orientation is with the edges of the cubeeither parallel or perpendicular to the direction of the magnetic fieldapplied during the heat treatment. In order to obtainia high percentageof products in which the massive crystal is so oriented, a seed crystalis planted close to one end of the bar during pressing. These seeds areso placed with respect to the well defined 100 crystal plane of thecrystal that upon growth thereof, there is obtained a massive crystal ofthe proper orientation. The seeded bars are then cycled through thesinteringfu-rnace as described hereinbefore with the seeded end of theba preferably entering the furnace first. After sintering, it is foundthat in at least 75% of the products, there is obtained a favorablecrystal orientation so that the resultant products have a BI-I max.value approaching the maximum theoretical value. I

The seeding crystals employed in this process can be obtained, forexample, by fracturing the sintered massive crystal products of thepresent invention. The specimens appear to fracture along the 100crystallographic face and the Seed crystals are placed in the compactswith this face properly oriented.

While the invention has been particularly described with reference tospecific Alnico compositions, it is to be understood that it is notlimited thereto. Any of the various compositions coming within theteachings of the above-mentioned Jonas patent can be employed, providedthe aluminum and, if necessary, any other lowmelting point componentwhich is present in a substantial amount, is employed in the form of analloy melting above th sintering temperatures and provided further thatthe pressed powder is sintered at a temperature approaching, but below,the melting point of the entire composition. It is further to beunderstood that the invention is not limited to binary aluminumpre-alloys but includes the addition of the aluminum in the form of anyhigh melting point alloy having no molten phase at sinteringtemperatures and consisting of two or more of the metal constituents ofthe Alnico compositions.

We claim:

1. The method of making a sintered anisotropic permanent magnet having amassive crystalline structure which comprises compacting a mixture ofpowdered metals including iron, nickel, cobalt and aluminum, thealuminum being present in the form of an alloy with an iron group metalhaving no molten phase at the sintering temperature of the mixture, saidaluminum alloy being selected from the group consisting ofaluminum-cobalt alloys containing from about 67 to 85 percent by weightcobalt, aluminum-nickel alloys containing from about 64.- to percent byweight nickel, and aluminum-iron alloys containing up to 18 percent byweight aluminum, and sintering the compact at a temperature of from 1200to 1400 C., which temperature is below that at which any molten phase isformed in said mixture.

2. The method of making a sintered anisotropic magnet having a massivecrystalline structure and a BH max. of at least about 5x10 whichcomprises forming a mixture of powdered iron, nickel, aluminum, cobaltand copper in which the aluminum is present in the form of analuminumcobalt alloy containing from 67 to about 85 percent by weight ofcobalt and sintering the formed mixture at a temperature of from 1200 to1400 C., said mixture containing from 16 to 30 percent cobalt, 12 topercent nickel, 6 to 11 percent aluminum, up to 3 percent copper, andbalance substantially all iron.

3. The method of claim 2 in which the aluminum-cobalt alloy containsabout percent aluminum, balance substantially cobalt and the sinteringtemperature is 1380 to 1385 C.

4. The method of making a sintered anisotropic magnet alloy whichcomprises mixing a plurality of metal powders essentially includingiron, nickel and aluminum, the aluminum being in the form of an alloywith an iron group metal which alloy contains no molten phase at thesintering temperature, compacting the mixture with a seed crystal withinsaid compact at a temperature of 1200 to 1400* C. and oriented in thedesired direction and sintering the compact to obtain a productcontaining massive crystals of the same orientation as the seed crystal.

5. The method of claim 4 in which the sintered alloy consists essentiallof 6 to 11 percent aluminum, 16 to percent cobalt, 12 to 20 percentnickel and about 3 percent copper, balance iron; and the aluminum isemployed in the form of an aluminum-cobalt alloy.

6. The method of claim 4 in which the sintered alloy consistsessentially of about 8.5 percent aluminum, 14 percent nickel, 25 percentcobalt, 3 percent copper, balance iron except for incidental impurities,and the aluminum is in the form of a powdered aluminum-cobalt alloycontaining about 25 percent aluminum and percent cobalt.

7. The method of making a sintered anisotropic magnet having a massivecrystalline structure and a BH max. of at least about 5 10 whichcomprises forming a mixture of powdered iron. nickel, aluminum, cobaltand copper in which the aluminum is present in the form of an alloy ofaluminum and an iron group metal having no molten phase below 1400" C.,said aluminum alloy being selected from the group consisting'ofaluminum-cobalt alloys containing from about 67 to 85 percent by weightcobalt, aluminum-nickel alloys containing from about 64 to percent byweight nickel, and aluminum-iron alloys containing up to 18 percent byweight aluminum, and sintering the formed mixture at a temperature offrom 1200 to 1400 C., said mixture containing from 16 to 30 percentcobalt, 12 to 20' percent nickel, 6 to 11 percent aluminum, up to 3percent copper, and balance substantially all iron.

ROBERT J. STUDDERS. DOLPH G. EBELING.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,733,752 Ramage Oct. 29, 19292,192,743 Howe Mar. 5, 1940 FOREIGN PATENTS Number Country Date 592,506Great Britain Sept. 19, 1947 OTHER REFERENCES Preparation of MetalSingle Crystals" by Holden, preprint by American Society for Metals(1949), pages 1244.

Ritzau: Wiss. Veroffentl. Siemens-Werke (Werkstofi' Sonderheft) 1940,page 37 as reported in Goetzel Treatise on Powder Metallurgy," vol. II,1950, pages 253 and 254.

1. THE METHOD OF MAKING A SINTERED ANISOTROPIC PERMANENT MAGNET HAVING AMASSIVE CRYSTALLINE STRUCTURE WHICH COMPRISES COMPACTING A MIXTURE OFPOWDERED METALS INCLUDING IRON, NICKEL, COBALT AND ALUMINUM, THEALUMINUM BEING PRESENT IN THE FORM OF AN ALLOY WITH AN IRON GROUP METALHAVING NO MOLTEN PHASE AT THE SINTERING TEMPERATURE OF THE MIXTURE, SAIDALUMINUM ALLOY BEING SELECTED FROM THE GROUP CONSISTING OFALUMINUM-COBALT ALLOYS CONTAINING FROM ABOUT 67 TO 85 PERCENT BY WEIGHTCOBALT, ALUMINUM-NICKEL ALLOYS CONTAINING FROM ABOUT 64 TO 80 PERCENT BYWEIGHT NICKEL, AND ALUMINUM-IRON ALLOYS CONTAINING UP TO 18 PERCENT BYWEIGHT ALUMINUM, AND SINTERING THE COMPACT AT A TEMPERATURE OF FROM 1200TO 1400* C., WHICH TEMPERTURE IS BELOW THAT AT WHICH ANY MOLTEN PHASE ISFORMED IN SAID MIXTURE.